It is almost universally desirable to enhance efficiency of devices. This is true for lamps and particularly for miniature lamps where it is desirable that virtually every photon emitted by the light source be projected by the lamp.
Similarly, it may be desirable to enhance the efficiency of an energy accepting device so that virtually every photon entering the device within a range of angles is absorbed by a meaningful light energy transducer such as a photodetector or means for utilizing the thermal energy obtained by absorbing light.
Reflective optics suitable for concentrating incident light on an energy absorber are described in various papers such as those by V. K. Baranov et al. in the Soviet Journal of Optical Technology. Vol. 33, No. 5, pp. 408-411 (1966) and Vol. 34, No. 1, pp. 67-70 (1967); D. E. Williamson "Cone Channel Condenser Optics", Journal of the Optical Society of America, Vol. 42, No. 10, pp. 712-715, (Oct. 1952); W. Witte, "Cone Channel Optics", Infrared Physics, Vol. 5, pp. 179-185, (1965); and M. Ploke "Light Conductors with Strong Concentration Effect", presented at DGOO convention, Bombay, India, (1966). Standard optics texts describe how to design reflective concentrating systems. Such papers deal primarily with reflective devices for concentrating energy incident on the device within a given angular range onto an absorber of such light. The reflective walls of the device and the absorbers are configured so that essentially every photon entering the device impinges on the absorber directly or after a limited number of reflections from the walls (usually one reflection). Techniques are described for enhancing concentration, namely enhancing the energy density illuminating the absorber.
Similar principles may be employed for designing high efficiency lamps, although the resultant geometry will differ from a practical absorber. In such an embodiment, a light emitting device may be placed adjacent to curved reflective walls so that virtually all of the light emitted by the device is projected by the lamp within a selected angular range.
The published literature describes how to select reflective surfaces for a concentrator such that extremal rays entering the concentrator at the maximum angle of acceptance will be retained within the concentrator and not reflected by its walls back out of the entrance aperture. For purposes of exposition, the angle of such extremal rays which are not reflected out of the concentrator is referred to as the cutoff angle.
A similar cutoff angle can be defined for the reflector of a lamp. For example, a miniature lamp may be in the form of a cup with reflective walls. A light emitting diode (LED) is mounted in the bottom of the cup. The cup walls are curved so that light emitted from the LED either passes directly through the opening or exit aperture of the cup or is reflected from a wall through the opening of the cup (opening and aperture are used substantially interchangeably herein since the opening of the cup is its optical aperture). The depth of the cup and the shape of its walls, particularly in the portion near the aperture of the cup, determine the cutoff angle of light projected from the cup. Thus, for an axisymmetric cup substantially all of the light is projected within a cone having an included half angle corresponding to the cutoff angle. With a well designed, toleranced and manufactured cup, there is very little light outside of this cone. Some light will usually be found outside of the cone due to surface irregularities and aberrations that distinguish real optical systems from ideal optical systems.
It is desirable to have the reflective cup of such a lamp filled with a transparent medium which has a refractive index better matched to the refractive index of the LED than air would be. This enhances the light output of the LED, and hence light output of the lamp. Typically, the cup may be filled with epoxy resin. Eventually the light emitted into the transparent medium is incident on an interface between that medium and air. It is highly desirable to enhance the transmission of light through that interface for obtaining maximum light output from the lamp. This invention addresses that desideratum.
The light from a lamp employing principles of nonimaging reflectors may need to be imaged or directed toward a selected field of view after being projected from the lamp. It is therefore desirable to provide means of association with the lamp for forming an optical image. This desideratum is also addressed by this invention.
It will also be found that principles of this invention may be applicable to devices which accept light energy as well as devices that project light energy.